Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01C—MEASURING DISTANCES, LEVELS OR BEARINGS; SURVEYING; NAVIGATION; GYROSCOPIC INSTRUMENTS; PHOTOGRAMMETRY OR VIDEOGRAMMETRY
  • G01C9/00—Measuring inclination, e.g. by clinometers, by levels
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B5/00—Measuring for diagnostic purposes; Identification of persons
  • A61B5/103—Measuring devices for testing the shape, pattern, colour, size or movement of the body or parts thereof, for diagnostic purposes
  • A61B5/11—Measuring movement of the entire body or parts thereof, e.g. head or hand tremor or mobility of a limb
  • A61B5/1116—Determining posture transitions

Definitions

• the invention relates to an accelerometer that measures acceleration in three dimensions, and in particular to methods for processing the measurements from the accelerometer.
• an object in three dimensional space has six degrees of freedom, translation along three perpendicular axes and rotation about three perpendicular axes.
• the motion indeed has six degrees of freedom.
• accelerometers that can measure accelerations along the three translational axes
• gyroscopes that can measure the rotations around the three rotational axes
• magnetometers that can measure the orientation of the object relative to an external magnetic field are used to monitor the six degrees of freedom of the object.
• the three dimensional accelerometer can only measure three possible degrees of freedom, and in order to measure six degrees of freedom, an electronic gyroscope is used.
• Algorithms are used to compensate for the rotation of the accelerometer relative to an external reference frame (such as a reference frame fixed relative to the Earth) which enables the measurement of the acceleration to be converted into the Earth reference system.
• an external reference frame such as a reference frame fixed relative to the Earth
• using gyroscopes has several disadvantages; firstly, gyroscopes are expensive and consume a lot of energy in comparison to an accelerometer or magnetometer, and secondly, the algorithms used to rotate the accelerometer reference system into the Earth reference system are computationally intensive.
• a method for estimating the orientation of an accelerometer in the absence of a gyroscope or other orientation sensor. It is a further or alternative object of the invention to provide a method of estimating the acceleration in a vertical direction of an external reference frame (such as the Earth) from the measurements from the accelerometer.
• a method for estimating the orientation of an accelerometer relative to a fixed reference frame comprising obtaining signals from the accelerometer, the signals indicating the components of the acceleration acting on the accelerometer along three orthogonal axes; identifying the axis with the highest component of acceleration; and determining the orientation of the accelerometer by determining the angle between the acceleration acting on the accelerometer and the axis with the highest component of acceleration.
• the angle, ⁇ , between the acceleration acting on the accelerometer and the axis with the highest component of acceleration is determined from
• a z is component of the acceleration along the axis with the highest component of acceleration
• a x and A y are the components of the acceleration along the other two axes.
• the method further comprises checking for local instability in an orientation determined in a particular sampling instant, i, by obtaining a set of signals from the accelerometer for a plurality of sampling instants around the particular sampling instant; and computing the variance of the norm of the components of the acceleration acting on the accelerometer along the three orthogonal axes for each of the set of signals.
• the step of computing the variance of the norm comprises calculating:
• ⁇ is a value selected from the range 15 m/s 2 to 20 m/s 2 .
• acceleration due to gravity is acting on the accelerometer.
• gravity acts in a known direction in the fixed reference frame, and the angle between the acceleration acting on the accelerometer and the axis with the highest component of acceleration provides an estimate of the orientation of the accelerometer relative to the known direction.
• a method for estimating the acceleration in a particular direction relative to a fixed reference frame from measurements of acceleration acting on an accelerometer, the accelerometer having an arbitrary orientation relative to the fixed reference frame comprising estimating the orientation of the accelerometer relative to the fixed reference frame as described above; and using the estimated orientation of the accelerometer to determine the acceleration in the particular direction from the measurements of acceleration.
• a method for estimating the acceleration in a vertical direction relative to a fixed reference frame from measurements of acceleration acting on an accelerometer, the accelerometer having an arbitrary orientation relative to the fixed reference frame comprising estimating the orientation of the accelerometer relative to the fixed reference frame as described above; and using the estimated orientation of the accelerometer to determine the acceleration in the vertical direction from the measurements of acceleration.
• an apparatus for estimating the orientation of an accelerometer relative to a fixed reference frame comprising processing means adapted to perform the methods described above.
• an apparatus for estimating the acceleration in a vertical direction relative to a fixed reference frame from measurements of acceleration acting on an accelerometer, the accelerometer having an arbitrary orientation relative to the fixed reference frame comprising processing means adapted to perform the methods described above.
• a computer program product comprising computer executable code that, when executed on a suitable computer or processor, is adapted to perform the methods as described above.
• the invention provides a method for calculating the tilt angle of the accelerometer without the need for a gyroscope or any other sensor, and a method for calculating the vertical acceleration in a fixed reference frame from the tilt angle.
• the movements of the accelerometer are slow (for example movements which have a vertical acceleration of no more than ⁇ 20m/s 2 ) the vertical acceleration calculated in accordance with the invention will be of a similar accuracy to that calculated using a system that includes a gyroscope and other sensors.
• Fig. 1 is a diagram illustrating the calculation of the orientation of an accelerometer from the measured acceleration
• Fig. 2 is a flow chart illustrating a method of estimating the orientation of an accelerometer
• Fig. 3 is a diagram illustrating an accelerometer attached to a user; and Fig. 4 is a set of graphs indicating the performance of the method according to the invention.
• Fig. 1 is an illustration of a measurement of an acceleration A measured by an accelerometer.
• the accelerometer measures the acceleration A acting on it in three dimensions, and provides signals indicating the acceleration A along three orthogonal axes (labelled x a , y a and z a ).
• the accelerometer When the accelerometer is attached to a person or other object that is capable of movement with respect to a fixed reference frame, it is possible for the orientation of the accelerometer to change with respect to the fixed reference frame.
• the acceleration A has components A x , A y and A z measured along the three axes respectively.
• the acceleration A experienced by the accelerometer will correspond substantially to that of gravity.
• the orientation of the accelerometer can be estimated by calculating the angle between the acceleration A and the axis of the accelerometer that has the highest magnitude of acceleration.
• step 101 the accelerometer measures the acceleration acting on the accelerometer, and provides signals indicating the components of the acceleration (A x , A y and A z ) along the three orthogonal axes of the accelerometer (x a , y a and z a respectively).
• step 103 the magnitudes of each component of the acceleration A are compared to identify the component with the highest magnitude.
• the axis (x a , y a or z a ) with the component with the highest magnitude is denoted z a ', and the other two axes are denoted x a ' and y a '.
• the accelerometer may not be attached to the object or person in this way (it may be that the y a axis corresponds most closely to the vertically oriented axis in the fixed reference frame).
• step 105 the angle between the acceleration A and the axis with the highest component of acceleration (z a ') is determined.
• the angle, ⁇ is given by:
• the angle ⁇ can be considered as indicating the orientation of the accelerometer.
• the accelerometer is free to move with respect to the fixed reference frame, it is desirable to check for local instability caused by rapid changes in the acceleration. In this way, it is possible to compensate for errors in the determined orientation caused by these rapid changes in acceleration.
• local instability is checked by computing the variance of the norm of the components of the acceleration A over a period of time.
• a number of signals are obtained from the accelerometer representing the acceleration at a number of sampling instants. These sampling instants preferably occur both before and after the sampling instant, i, at which the orientation of the accelerometer is calculated.
• the variance of the norm of the components of the acceleration A are calculated using:
• ⁇ is a value that indicates a rapid change in acceleration
• ⁇ is a value selected from the range 15-20 m/s 2 . In an even more preferred embodiment, ⁇ is 17 m/s 2
• a and b are 10.
• Fig. 3 shows an accelerometer 2 attached to a person 4.
• the person 4 is part way through a sit to stand transfer, and the accelerometer 2 is oriented at an angle ⁇ from the vertical.
• the axis with the highest component of acceleration (A z ) is shown.
• the acceleration in the vertical direction is calculated from:
• ace _ vert (A 2 - g cos ⁇ )cos ⁇ + g, if ⁇ > 0 or there is local instability (3)
• acc vert (g cos ⁇ - A 2 )cos ⁇ + g, if ⁇ ⁇ 0 or there is no local instability (4) where g is the magnitude of the acceleration due to gravity in the vertical direction. It will be appreciated that ⁇ ⁇ 0 in Figs. 1 and 3.
• Fig. 4 is a set of graphs showing some test data used to validate the methods according to the invention.
• the first graph in Fig. 4 shows the signals representing the acceleration along each of the axes of the accelerometer; the second graph shows the vertical acceleration calculated using the accelerometer and a gyroscope; the third graph shows the vertical acceleration as estimated by the methods described herein; and the fourth graph shows the relative error between the second and third graphs.
• the methods according to the invention result in an error of generally less than 5% when compared to methods of determining a vertical acceleration in which gyroscopes are used.
• the methods for calculating the orientation and vertical acceleration can be used in any application where accelerometers and gyroscopes are normally used, and in particular can be used in devices that detect when a person has fallen, or is about to fall. As described above, the methods can also be used to determine the vertical acceleration involved in a person standing up from a sitting position.
• a computer program may be stored/distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems. Any reference signs in the claims should not be construed as limiting the scope.

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• Measurement Of The Respiration, Hearing Ability, Form, And Blood Characteristics Of Living Organisms (AREA)
• Length Measuring Devices With Unspecified Measuring Means (AREA)
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• 2009
  • 2009-09-18 EP EP09787235A patent/EP2331908A2/de not_active Withdrawn
  • 2009-09-18 CN CN200980137008XA patent/CN102159920A/zh active Pending
  • 2009-09-18 WO PCT/IB2009/054086 patent/WO2010035191A2/en not_active Ceased
  • 2009-09-18 US US13/063,938 patent/US20110172951A1/en not_active Abandoned
  • 2009-09-18 BR BRPI0913711A patent/BRPI0913711A2/pt not_active IP Right Cessation
  • 2009-09-18 JP JP2011527452A patent/JP2012503194A/ja not_active Withdrawn

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